This invention relates to oscillations, and particularly but not exclusively to oscillators for use in currency validators, especially coin validators.
It is known to test coins using inductances in the form of coils positioned in proximity to a coin path, and driven by oscillators. As the coin passes the coil, the performance of the oscillator circuit is monitored to determine the effect of the coin and thus provide a measurement of the coin""s properties. The influence of the coin on the frequency, amplitude or phase of the oscillations may be monitored. The measurement is normally based on the change in a monitored parameter, for example the difference or ratio between the parameter when the coin is absent and that when the coin is present.
The influence of the coin on the measured parameter is a function of frequency. See, for example, GB-A-1 397 083 . It is known to subject the coin to oscillations at two separate frequencies and measure the effect at both frequencies in order to derive further information about the coin. This is particularly useful for clad coins (formed e.g., by an outer material rolled on top of an inner material, or by plating the inner material), as higher frequencies will be less influenced by the inner material and more influenced by the outer material. The inner material of a clad coin is sometimes referred to as the xe2x80x9cbulkxe2x80x9d or xe2x80x9ccorexe2x80x9d material.
GB-A-2 069 211 discloses a coin validator in which a coil on one side of a coin path is driven at a combination of two frequencies, and a receiving coil at the opposite side of the coin path is coupled to means for detecting the influence of a coin on the amplitude of the received signal at the two different frequencies. Monitoring means are connected to the receiving coil through filter circuits to separate the different frequencies. However, this arrangement does not permit a variation in the oscillation frequency as a result of the presence of the coin. Furthermore, the use of a transmit/receive arrangement is often undesirable, particularly as the received signal strength varies by very large amounts, especially with magnetic coins. It would also be desirable to avoid the use of filters.
Aspects of the present invention are set out in the accompanying claims.
According to a further aspect, two self-excited oscillators operate at different frequencies and share at least one common inductance. Such an arrangement can be used in a coin validator for testing a coin, in which the value of the inductance is influenced by a coin under test.
It is known to use a pair of coils, for determining the material content of a coin. By using an oscillator coupled to both coils, this arrangement produces significantly more gain than a single coil, due to the mutual inductance between the coils. However, because both coils are running at the same frequency, only a single material measurement is made.
According to a preferred aspect of the invention, the two self-excited oscillators share a pair of coils, positioned one on each side of a coin path. Operating both coils concurrently at different frequencies is particularly useful for determining the material content of the coin at different depths within the coin.
Crosstalk between the oscillators could be avoided by appropriate filter circuits. However, in a particularly preferred embodiment, the oscillators are configured so that oscillations at each frequency appear at a node which constitutes a signal null for the other frequency. This isolates the frequencies without requiring additional filter circuits.
In the preferred embodiment, a signal null for one frequency is produced because, at that frequency, there is a very low a.c. impedance to a.c. ground. A signal null for another frequency is produced because the other frequency is applied to the node in equal and opposite amounts.